Modern aircraft secondary power systems are becoming large with increasing demands for electric power and large power for starting the prime propulsion engines. These aircraft have a gas turbine powered auxiliary power unit (APU) on board to provide electrical, hydraulic and pneumatic power for operating systems aboard the aircraft when the propulsion engines are shut down as well as to provide power for starting the propulsion engines. Typically, pneumatic power, provided by a load compressor within the APU, has been used to start the propulsion engines. The load compressor has also been used to provide compressed air for the environmental control system (ECS) aboard the aircraft before and after the propulsion engines are started.
The main engine start (MES) requires much more compressed air from the APU than the ECS load. Since the load compressor is used for both the MES mode and the ECS mode, the APU is sized well above the needs by the MES requirement. The APU is thus oversized for the ECS mode and it is heavier, more costly and less efficient than if it were sized to match the ECS load alone. The heavier APU and its support structure detract from the aircraft fuel load or payload. The excess APU weight therefore shortens the range of the aircraft or reduces its payload.
A co-pending application having the same inventorship and owned by the assignee of this application, describes a combined power start system using pneumatic power combined with concurrent APU electric power, hydraulic power, or both electric and hydraulic power to start the main propulsion engines. This approach results in sizing the APU pneumatic system more consistently with both MES and ECS need, avoiding excess weight that would otherwise result from over-sizing the pneumatic portion of the ECU.
When the APU electric power is used to supplement or replace pneumatic power for MES, it has been usual practice to rectify alternating current (AC) power generated by the APU generator or starter/generator to direct current (DC) power, and then convert the rectified DC power back to AC power at a preferred frequency that is suited to the rest of the aircraft electrical power system. For constant frequency aeronautical electrical power systems, 400 Hz is generally preferred.
A problem with this approach is that much power must be converted, and since electrical conversion equipment is of limited efficiency, significant power conversion loss occurs and this power loss generates heat that must be dissipated. Excess power must be generated to cover the power loss and excess equipment must be provided to dissipate the resulting heat. These factors lead to increased system weight and cost that negatively impact the economics, payload and range of the aircraft.